TPS22998 datasheet(12/21 Pages) TI

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8.2.2 Detailed Design Procedure

When the switch enables, the charge up the output capacitance from 0 V to the set value (1.8 V in this example).

This charge arrives in the form of inrush current. Calculate inrush current using Equation 1.

Inrush Current = CL × dVOUT/dt

(1)

Where:

• CL is the output capacitance.

• dVOUT is the change in VOUT during the ramp up of the output voltage when device is enabled. Because

rise time is 10% of VOUT to 90% of VOUT, this is 80% of the VIN value.

• dt is the rise time in VOUT during the ramp up of the output voltage when the device is enabled.

The TPS22998 offers an adjustable rise time for VOUT, allowing the user to control the inrush current during turn

on. Calculate the appropriate rise time using the design requirements and the inrush current equation as shown

below.

1A = 470 µF × (1.8 V × 80%) / dt

(2)

dt = 677µs

(3)

To ensure an inrush current of less than 1 A, a CT setting that yields a rise time of more than 677 µs must be

chosen. By pulling the CT pin high, a rise time of 900 μs is selected, limiting the inrush current to below 1 A.

8.2.3 Application Performance Plots

The below scope shot shows the TPS22998 turning on into a 470-μF load with the CT pin tied to VBIAS.

Figure 8-2. TPS22998 Turn On into 470 μF (CT = VBIAS)

TPS22998

SLVSG05 – OCTOBER 2021

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TPS22998 Datasheet(PDF) 12 Page - Texas Instruments